My New Pi Won't Boot With My Previous microSD Card?
The shiny new Pi board features a shiny new processor. This means that previous OS images that you ran on your older Pi models will not run on the new processor!
Raspbian is the first OS to be updated, as the Raspberry Pi Foundation provided a new release on the same day as the Pi board. You can find that new version here. Please also see this link for general instructions on how to avoid the common pitfalls of writing the image file to the microSD card.
Other OS's will be updated, but they didn't have advance notice of the new board model, so they now have to catch up. Please check with the developer of the OS you're interested in to see if they list a release date.